Protective garments for wear in rain and other wet conditions should keep the wearer dry by preventing the leakage of water into the garment and by allowing perspiration to evaporate from the wearer to the atmosphere. In the past, and through a long history of rainwear development, truly waterproof materials have not allowed the evaporation of perspiration, so that a wearer who is physicaly active, becomes sweat soaked. "Breathable" materials that do permit evaporation of perspiration, have tended to wet through from the rain, and they are not truly waterproof. Oilskins, polyurethane coated fabrics, polyvinyl chloride films and other materials are waterproof but do not allow satisfactory evaporation of perspiration.
Fabrics treated with silicone, fluorocarbon, and other water repellants usually allow evaporation of perspiration but are only marginally waterproof; they allow water to leak through them under very low pressures, and usually leak spontaneously when rubbed or mechanically flexed. Rain garments must withstand the impingement pressure of falling and wind blown rain and the pressures that are generated in folds and creases in the garment.
It is widely recognized that garments must be "breathable" to be comfortable. However, it is not necessary that air pass through the garment for it to be comfortable, only that water vapor from perspiration be transmitted from inside to outside so that undergarments do not become wet and so that the natural evaporative cooling effect can be achieved. Breathability and ability to transport interior moisture vapor to the external environment are used interchangeably in this discussion.
The transport of water through a layer can be achieved in a number of ways. Wicking is the most common when large quantities of moisture are to be transferred. Wicking materials are hydrophilic in that a drop of water placed on the surface of these materials forms an advancing water contact angle of less than 90 degrees so that they wet spontaneously. They are also porous with pores that interconnect to make complete pathways through the wicking material. Liquid water moves by capillary action from interior surface to exterior surface where it evaporates. Although some wicking materials may resist pressure induced flow of liquid water through them due to the tortuousity and length of flow path, they readily transport water by capillary action from the exterior surface to the interior surface and so are unsuitable for rain material. The comfort attributed to cotton garments in warm climates results from its ability to transport water to the exterior surface where it can readily evaporate and provide cooling. Another natural wicking material is leather which owes its great comfort to breathability via wicking.
A recent invention (U.S. Pat. No. 3,953,566) has provided porous membranes that satisfy the two comfort requirements of being waterproof while also being permeable to the flow of water vapor. For rainwear, these membranes are usually laminated to fabrics for mechanical protection and style. The membranes are inherently hydrophobic and contain very small pores that resist the entry of liquid water even at substantial pressures or when rubbed or flexed, but readily allow the flow of gases, including water vapor. Unlike wicking materials, breathability is achieved by evaporation of liquid water inside the garment or on the inner surface of the membrane followed by gaseous flow or diffusion of water vapor through the membrane to the outside.
However, when these new garments are worn for strenuous activities causing the wearer to perspire copiously, surface active agents in the perspiration gradually penetrate the hydrophobic membrane, coat its interior surfaces and cause it to lose its waterproof characteristics and become a wicking material. In order to restore waterproofness, the garment must be cleaned to remove the surface active contaminates. In practice this is a drawback to widespread commercial acceptance of such garments.